The present invention relates to a numerical control system, and more particularly to a numerical control system which is suitably applied to a numerical control machine tool wherein a tool is moved or rotated in the directions of three X, Y and Z (orthogonal coordinate system) axes and in the directions of two B and C (spherical coordinate system) axes so as to subject a workpiece to three-dimensional machining.
In a machine tool, for example as found in a machining center, a tool or a table is rotated in the vertical rotational direction (B-axial direction) and the horizontal rotational direction (C-axial direction) so as to control the axial direction of the tool relative to a workpiece or the table (the axial direction being the tool axis direction), and it is also moved in the directions of the three X, Y and Z axes so as to subject the workpiece to a desired machining operation.
In general, in such 5-axis control, the position Q(x, y, z) of the center of rotation of the tool and the position of the tool axis; that is the angular position (b, c) of a rotational angle are entered from a command tape as numerical control (hereinafter "NC") command data. When supplied with the NC command data, a pulse distribution circuit within the NC system executes pulse distribution computations to rotate the tool in the B- and C-axial directions so as to bring the axial direction of the tool into agreement with a commanded tool axis direction, for example, normal direction to the workpiece. The pulse distribution circuit also moves a tool holder as well as the tool in the X, Y and Z directions so as to subject the workpiece to the programmed three-dimensional machining.
In this regard, the command tape for the 5-axis control is usually prepared as follows. Firstly and with reference to FIG. 1, the coordinates (X, Y, Z) of the position P of the front end of the tool, as found by a main processor, the vector (I, J, K) of the tool axis direction at the front end position (for example, normal to the workpiece) as found by the main processor, and a length l from the position Q of the center of rotation of the tool to the position P of the front end of the tool, are input to a post-processor. The position Q (x, y, z) of the center of rotation of the tool and the position (b, c) of the tool axis are found by the post-processor. Lastly, these values x, y, z, b and c are punched in a tape as the command data, to obtain the predetermined command tape.
When using a tape prepared using the above method, when the length l of the tool changes, this tape cannot be used and a separate new tape must be made, even when the shape to be machined is identical. Moreover, in a machining operation in which fine finishing is performed after rough finishing, two tapes, one for the rough finishing and one for the fine finishing, must be prepared, and a single tape cannot be used for both the rough finishing and the fine finishing.